As for dyeing with a fluorescent brightener, it is usual that conventionally available powdery or granular dye is once dissolved in hot water and is subjected to dyeing. On the other hand, in paper factories or dyeing factories, automation and FA have progressed and there has been a demand for liquid products which are able to cope with automated measurement systems.
Aqueous solution of fluorescent brightener has a disadvantage that, due to the presence of inorganic salts which are by-produced during the manufacture of fluorescent brightener, stability at low temperature and at high temperature is low whereby crystals, etc. are separated out. In order to prevent that, inorganic salts contained in aqueous solution of fluorescent brightener are decreased using semipermeable membrane or like to enhance the stability. For example, JP-A-58-65760 and JP-A-60-158266 disclose a method where concentration of salts in aqueous solution of fluorescent brightener of a stilbene derivative is decreased to prepare a concentrated aqueous solution is manufactured.
With regard to a method where no semipermeable membrane is used, a method, for example, JP-A-58-222156 discloses where insoluble or hardly-soluble metal salt of stilbene derivative is made to react with lower hydroxylamine in the presence of carbonate ion and insoluble substance is removed from the reaction mixture to prepare a stable concentrated aqueous solution containing the stilbene derivative as a fluorescent brightener.
Further, JP-A-57-123262 discloses that a tetraalkylammonium compound such as tetramethylammonium, tetraethylammonium or tetrapropylammonium is added to aqueous solution of sodium salt of stilbene derivative for example to convert the sodium salt to tetraalkylammonium salt to prepare a fluorescent brightener mainly comprising tetraammonium salt and thereby a part of specific dyes becomes to exhibit higher solubility than sodium salt, etc. Furthermore, JP-A-62-273266 discloses a method as an improved method of the method disclosed in JP-A-57-123262, wherein sodium salt of stilbene derivative is changed to a salt with a tetraammonium compound having hydroxyl group such as choline, desalted using a compressed filtering apparatus and then a solubilizing agent such as urea is added thereto to manufacture a stable concentrated aqueous solution containing the stilbene derivative as a fluorescent brightener.
Incidentally, the stilbene derivative used in the present invention and represented by the formula (1) mentioned later is disclosed in JP-A-58-22156, British Patent 1,243,479 and Belgian Patent 719065 for its structure. Further, in British Patent 1,243,479 and Belgian Patent 719065, the compound of the formula (1) is also disclosed together with other stilbene derivatives and the synthetic method and the use as fluorescent brightening for paper, etc are disclosed. However, in those patents, there is no disclosure relating to preparations of a dyeing solution (including a coating liquid) or a concentrated aqueous solution which is prepared in such a manner that the solution is able to be directly used for dyeing.
Since anionic dye is used in an ink for ink jet printers using a water-soluble dye, there are many cases in a high quality-recording paper for ink jet printers that, in order to suppress the blotting of ink on the paper, a cationic ink fixing agent or water resisting agent is used during the manufacture of the recording paper. In addition, since clear coloration is demanded for the recording paper, there has been a demand for a degree of whiteness of high quality. Therefore, recording paper dyed in high whiteness is manufactured using various kinds of fluorescent brighteners. With regard to the fluorescent brightener used at that time, an anionic water-soluble fluorescent brightener is frequently used due to the characteristic of the manufacturing process. However, when the above-mentioned cationic fixing agent and anionic fluorescent brightener are used together, the ink fixing agent is combined with the fluorescent brightener in a solution to make them insoluble in water and, as a result, there is a disadvantage of causing the phenomena that crystals are separated out and that high whiteness demanded in the dyeing is not achieved.
As mentioned above, it was not possible to prepare any stable concentrated aqueous solution containing high concentration of stilbene derivative used for fluorescent brightening of cellulose such as paper, pulp or cotton unless excessive steps for the purpose of desalting, etc. are applied. Thus, excessive step, solubilizing agent, etc. are always necessary, for example, as shown in JP-A-58-65760that desalting with semipermeable membrane which requires time and cost is carried out; as shown in JP-A-58-222156 that sodium salt of stilbene derivative is once converted to an insoluble salt and then converted to a lower hydroxylamine salt and the precipitated inorganic salt is removed; or, as shown in JP-A-62-273266 that, after sodium salt of stilbene derivative is once converted to a salt with choline or the like, desalting is conducted and then a solubilizing agent is added to give a stable concentrated aqueous solution. Further, in fluorescent brightening of high quality recording paper for ink jet printers, a fluorescent brightener having a high fluorescent brightening effect is demanded even when a cationic fixing agent is used together. Therefore, there has been a demand for developing a concentrated aqueous solution which contains a high concentration of fluorescent brightener such as stilbene derivative, shows excellent storage stability at low and high temperatures, is able to be manufactured easily and at low cost and further satisfies the following requirements that
1. it shows an excellent brightening effect when it is used for fluorescent brightening for cellulose, etc.;
2. it achieves an excellent fluorescent brightening effect even when used together with cationic ink fixing agent for fluorescent brightening of recording paper, etc. for ink jet printers;
3. it has excellent fixing property and water fastness property when used for fluorescent brightening of recording paper, etc. for the ink jet printers; etc.